Conventionally, a method using a twisted-blade-shaped static mixer element 101 mounted in a pipe as illustrated in FIG. 12 has been common as a method for homogeneously mixing a fluid flowing in the pipe (see, for example, Patent Literature 1). Typically, the static mixer element 101 has a structure in which a plurality of minimum unit members, each of which is obtained by twisting a rectangular plate at 180 degrees about the longitudinal axis of the rectangular plate, are connected together in series so that the directions of the twisted minimum unit members are alternately different. The static mixer element 101 is arranged in a pipe 102, male connectors 103 are attached to both ends of the pipe 102, flares 105 are mounted, and clamping nuts 104 are screwed, whereby a static mixer is formed. In such a case, the outer diameter of the static mixer element 101 is designed to be approximately equal to the inner diameter of the pipe 102 to effectively stir fluid.